Conventional comb-line filters comprise cylindrical or rectangular metal pieces, which form at least a pair of resonators. Each of the resonators is respectively connected to a capacitor, which can be adjusted, for example, with a screw. The conventional comb-line filters suffer from the disadvantages of being relatively bulky in their physical dimension and are difficult to be mass-produced.
Relatively recently, planar type comb-line filters have been developed which are substantially smaller in dimension and can be mass-produced relatively easily. The planar comb-line filters are made by coating relatively thick films of an appropriate material on a substrate. However, it was found that, because of the substantially reduced distance between the pair of capacitors respectively connected to the resonators, significant "adjacent capacitive-coupling" effect has been observed which has become a major deterring factor of the planar comb-line filters. The adjacent capacitive-coupling effect can increase the passband bandwidth of the filter and adversely affect the transmission characteristics of the filters, thus causing the filters to be unable to meet the design requirement. In order to reduce such an undesirable effect, planar comb-line filters are typically designed to have resonators whose electrical length is greater than 50.degree. (i.e., 50/360 of the wavelength at which the filter is designed to operate). By increasing the electrical length of the resonators, the required capacitance of the capacitors can be decreased accordingly, thus reducing the adjacent capacitive-coupling effect. However, increasing the length of the resonator, which necessitates the increase in the overall dimension of the planar comb-line filters, may be defeating the very purpose of developing the compact-sized planar comb-line filters.
If the length of the resonators is not increased, the adjacent capacitive-coupling effect becomes appreciable. FIGS. 1a-1c show schematic top views of the various layers, the top layer, the first layer, and the bottom layer, respectively, of a typical planar comb-line filter. Both the top layer and the bottom layer are grounded metal plates which are separated by a distance of about 1 mm. The first layer (i.e., the first layer immediately below the top layer) consists of two resonators. Each resonator has a small protruded portion for serving as input or output. In FIG. 1b, the right-handed side of the resonator is grounded while its left-handed side is connected to a capacitor.
FIG. 2 is a plot of transmission coefficient, S.sub.21 (dB) vs. frequency, F (GHz) for a planar comb-line filter under ideal conditions. The simulation was done using an industrial standard full wave electromagnetic field simulation program under the hypothetical condition of a pair of ideal capacitors with zero adjacent capacitive-coupling. Each capacitor has a capacity of 22.6 pF. The length of the resonators in the ideal planar comb-line filter has been reduced to 26.5.degree. electrical length, and the passband has a central frequency of 947.5 MHz (or 0.9475 GHz).
However, the results can become quite different if the assumption of ideal condition is breached. FIGS. 3 and 4 are simulated plots of transmission coefficient, S.sub.21 (dB) vs. frequency, F (GHz) for two real life planar comb-line filters. Both planar comb-line filters have a pair of capacitors with the same capacity of 22.6 pF, however, they are arranged differently. As shown in FIGS. 3 and 4, both cases show a very significant bifurcation of the response curve. They also show increased bandwidth of the passband. The coupling capacities between the adjacent capacitors in FIGS. 3 and 4 are determined to be 5.5 pF and 2.4 pF, respectively. The comb-line filters as shown in FIGS. 3 and 4 also exhibit relatively high insertion loss at passband, and relatively low attenuation at stopband. Both are undesirable filter characteristics which are results of reduced filter dimension.
The above described problem was also discussed in U.S. Pat. No. 5,311,159 (the '159 patent). In order to provide miniaturized bandpass type filter which can be used in a frequency band more than about 1.5 GHz, the '159 patent devised a tri-plate line which is constructed from a resonance element formed by intervening dielectrics between one pair of ground conductors. The length of the line is adjusted to about 1/4 wave-length (or 90.degree. electric length). Then a plurality of resonators are combined to form a bandpass filter. While the '159 invention may have ameliorated the coupling problem of the resonators, it is relatively complex in design and would substantially increase the cost of making bandpass filters.
U.S. Pat. No. 4,963,843 disclosed a comb-line stripline filter which includes a number of conductive strips, each being connected to ground on one end and capatively loaded to ground at the other end. While the '843 invention solved some of the capacitive coupling problems, the results are not totally satisfactory; the electrical length of the resonators is generally set to about 75.degree.. Thus the '843 invention could not provide the desired miniaturization for today's portable communication needs.
At the present time, there are no comb-line filters which are compact in size, can be manufactured relatively easily and inexpensively, and provide desired frequency response.